Predictability of the rainy season onset date in Central Highlands of Vietnam

Abstract

In this study, different criteria to determine the rainy season onset date (RSOD) and its predictability in the Central Highlands (CH) of Vietnam are investigated. Using daily rainfall amounts from 10 meteorological stations for the period 1981-2015, four criteria to determine the RSOD were tested in order to select the one that most reasonably depicts the climatology of the RSOD over the CH. Results show that the RSOD varies strongly from year to year. In the long-term mean, the onset starts first in southern parts of the CH and then progresses to the northern parts. The earliest onset date is around the beginning of April, and the latest in the second half of May. The average RSOD is on 28 April with a standard deviation of 14 days. Thus, the RSOD is distinct from the summer monsoon onset, namely, leading the mean summer monsoon onset by about 3 weeks in some years. In terms of remote influences, the RSOD in the CH has a high correlation with the El Niño-Southern Oscillation phenomenon, with most RSODs being later during El Niño years, while being earlier during La Niña years. The RSOD in the CH also shows high correlations with sea surface temperatures (SSTs), 850-hPa zonal winds (U850) and mean sea level pressures (PMSL) over certain regions of the Pacific and Indian Oceans. Building on these relationships, the RSOD can be successfully predicted using large-scale fields of SST, U850 and PMSL as predictors in two different approaches, namely the field mean and principal component analysis. This suggests that the overall approach can also be applied in predicting the RSOD in the CH on sub-seasonal to seasonal timescales. K E Y W O R D SCentral Highlands, predictability, rainy season onset, seasonal prediction, Vietnam

The Second Rainy Stage Onset in the Central Highlands of Vietnam

“…Coincidentally, both the FRSOD and SRSOD in normal years are identical to those in the 28 years climatology, with fewer rainfall differences for each rainy spell, which suggests that the 28 years climate mean might nearly reach the climatic norm with the exception of the RSRD and SRSRD. Although the FR-SOD in each category is in early May, Pham-Thanh et al (2020) stated that an RSOD over the CH between late April and early May seems to be more reasonable, not to mention using different criteria and datasets.…”

The Performance of ECMWF Subseasonal Forecasts to Predict the Rainy Season Onset Dates in Vietnam

“…Among rainfall characteristics, the rainy season onset date (hereafter denoted as RSOD) has received major attention because of its important implications not only for agriculture but also for society due to its impact on electricity generation, water management, and human health (e.g., Bombardi et al 2020). Furthermore, changes in the starting dates of the rainy season or monsoon were documented for Vietnam (e.g., Kajikawa et al 2012;Ngo-Thanh et al 2018;Pham-Thanh et al 2019). Therefore, a reliable prediction of RSODs is one aspect to improve decision-making across several social sectors.…”

“…Statistical predictions of RSODs commonly depend on the influence of the large-scale circulation on local-scale rainfall characteristics (e.g., Omotosho 1992;Omotosho et al 2000;Lala et al 2020). For the south and the Central Highlands of Vietnam, previous studies suggested that multivariate linear regression approaches, which use large-scale circulation features such as pressure, moist static energy gradients, outgoing longwave radiation, and wind fields as predictors, provide skillful predictions of RSODs on seasonal time scales (Pham et al 2010;Pham-Thanh et al 2019).…”

Characteristic of the Regional Rainy Season Onset over Vietnam: Tailoring to Agricultural Application

“…Therefore, the date of onset of the rainy season is of particular importance to the agriculture sector in Vietnam. However, despite the fact that the investigation of rainy season onset dates has a huge practical importance for both agriculture and economy, targeted studies evaluating the timing of the rainy season in Vietnam have been very limited [8,9,10].…”

“…Studies investigate the climatology, trends and the predictability of rainy season onset dates in southern Vietnam [8] and the Central Highlands region [9] of Vietnam. Both studies also explored prediction of rainy season onset dates based on statistical modeling using the relationship between local rainy season onset dates and large-scale atmospheric variables (pressure and moist static energy gradients, outgoing long-wave radiation, wind fields and mean sea level pressures) over certain regions.…”